The present invention relates generally to the production of curved, tempered sheets of glass and, more particularly, to an improved method of and apparatus for bending and heat treating relatively thin glass sheets.
Bent sheets of glass are commonly used as glazing closures in vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures dictated by the configuration and size of the openings and the overall styling of the vehicle. Additionally, it is necessary that bent glass sheets utilized as glazing closures in vehicles be tempered to increase their resistance to damage resulting from impact and, in the event of breakage, to fragment into relatively small harmless particles as opposed to the large, jagged, potentially dangerous pieces otherwise resulting from untempered glass sheets when broken. Further, it is important that the bent and tempered glass sheets meet stringent optical requirements and that the viewing area of glazing closures be free of surface defects and optical distortions that would interfere with clear vision therethrough.
Generally, the commercial production of curved, tempered sheets of glass for such purposes includes heating pretrimmed, flat sheets of glass to the softening temperatures thereof, bending the heated sheets to a desired curvature between a pair of complementary mold sections and then chilling the bent sheets in a controlled manner to a temperature below the annealing range of glass. In a mass production operation, the above operations are carried out successively while the sheets of glass are being advanced substantially continuously along a fixed path including a heating area, a bending area and a chilling or tempering area whereby the heat initially imparted to the sheet to bring it to the proper bending temperature can also be utilized in the final heat treating or tempering operation. To achieve a satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level upon entering the tempering area. The residual heat remaining in glass sheets of conventional thicknesses, such as those having thicknesses ranging from 0.235 inch to 0.255 inch for example, generally is above such predetermined minimum level after bending for immediate advancement into the tempering area.
However, in recent years, considerable emphasis has been placed on the use of thinner glass sheets for automotive glazing purposes, the thicknesses thereof preferably ranging from about 0.125 inch to 0.156 inch for example. While the process described above is admirably suited for the mass production of the thicker glass sheets, the same process for the production of relatively thinner glass sheets has not been successful. This is due to the rapid loss of heat in the thin sheets during bending which is occasioned by the relatively cool bending tools and cooler ambient atmosphere of the bending area, which decreases the temperature of the glass sheets to a level below the minimum temperature at which adequate tempering can be effected. On the other hand, overheating the thin sheets of glass prior to bending to compensate for the rapid subsequent loss of heat during bending tends to degrade the surface quality of the finished glass as a result of heat stains, pitting, and the like. Also, such initial overheating renders the sheets extremely pliable with attendant loss of the deformation control necessary to maintain the bent sheets within the close tolerances dictated by automobile design and styling requirements.